Weighing the Giants IV: Cosmology and Neutrino Mass

Abstract: We employ robust weak gravitational lensing measurements to improve cosmological constraints from measurements of the galaxy cluster mass function and its evolution, using X-ray selected clusters detected in the ROSAT All-Sky Survey. Our lensing analysis constrains the absolute mass scale of such clusters at the 8 per cent level, including both statistical and systematic uncertainties. Combining it with the survey data and X-ray follow-up observations, we find a tight constraint on a combination of the mean matter density and late-time normalization of the matter power spectrum, $\sigma_8(\Omega_m/0.3)^{0.17}=0.81\pm0.03$, with marginalized, one-dimensional constraints of $\Omega_m=0.26\pm0.03$ and $\sigma_8=0.83\pm0.04$. For these two parameters, this represents a factor of two improvement in precision with respect to previous work, primarily due to the reduced systematic uncertainty in the absolute mass calibration provided by the lensing analysis. Our new results are in good agreement with constraints from cosmic microwave background (CMB) data, both WMAP and Planck (plus WMAP polarization), under the assumption of a flat $\Lambda$CDM cosmology with minimal neutrino mass. Consequently, we find no evidence for non-minimal neutrino mass from the combination of cluster data with CMB, supernova and baryon acoustic oscillation measurements, regardless of which all-sky CMB data set is used (and independent of the recent claimed detection of B-modes on degree scales). We also present improved constraints on models of dark energy (both constant and evolving), modifications of gravity, and primordial non-Gaussianity. Assuming flatness, the constraints for a constant dark energy equation of state from the cluster data alone are at the 15 per cent level, improving to $\sim 6$ per cent when the cluster data are combined with other leading probes.

Overview of "Weighing the Giants IV: Cosmology and Neutrino Mass"

The paper "Weighing the Giants IV: Cosmology and Neutrino Mass" by Adam B. Mantz et al. addresses cosmological constraints utilizing weak gravitational lensing and X-ray measurements of galaxy clusters. Specifically, the research refines constraints on the matter density, amplitude of matter fluctuations, dark energy models, and neutrino mass using robust measurements from X-ray selected clusters in the ROSAT All-Sky Survey.

Summary of Methods and Results

The authors employ weak lensing measurements to enhance the accuracy of cosmological parameter constraints obtained from the observed galaxy cluster mass function and its evolution. This approach effectively constrains the absolute mass scale of galaxy clusters to an 8% precision, incorporating statistical and systematic uncertainties.

The key numerical result of the study is a refined constraint on the combination of the mean matter density (\Omegam) and the normalization of the matter power spectrum (\sigma_8), with a combined parameter $\sigma_8(\Omegam/0.3)^{0.17}=0.81\pm0.03$. This signifies a factor of two improvement in precision over previous studies, attributed mainly to the new lensing analysis used for mass calibration.

Additionally, the study shows that there is no compelling evidence for non-minimal neutrino mass when combining cluster data with Cosmic Microwave Background (CMB) data from both WMAP and Planck, as well as supernova and baryon acoustic oscillation (BAO) measurements. This remains valid independent of the choice of all-sky CMB dataset or the consideration of other recent observations, such as B-modes detection.

The research also improves constraints on various cosmological and astrophysical models, including dark energy models with constant or varying equations of state, modifications of gravity, and primordial non-Gaussianity.

Implications

This research has significant implications both theoretically and practically. The innovative integration of weak lensing data with traditional X-ray mass estimations enhances our understanding and measurement accuracy of fundamental cosmological parameters. The improved precision in determining \Omegam{} and \sigma_8{} strengthens the consistency checks between observed and predicted values, particularly when validated against CMB measurements.

The absence of evidence for additional neutrino mass beyond the minimal set by neutrino oscillation experiments underscores the challenges in detecting subtle cosmological signals with precision cosmology. Future surveys with increased sensitivity and resolution may further test this non-detection.

Additionally, the constraints on dark energy and alternative models of gravity have profound implications for understanding the accelerated expansion of the Universe. By bounding these models, the study aids in refining theoretical predictions and guiding future observational strategies.

Future Developments

Looking into the future, the blend of gravitational lensing techniques with broader optical, X-ray, and other observation windows promises unprecedented insights into the mass-energy content of the Universe. This approach anticipates enhancing the robustness of cosmological datasets by reducing systematic uncertainties associated with cluster mass measurements.

The continuation and expansion of work like the "Weighing the Giants" series are pivotal for interpreting forthcoming results from new and planned observatories. Gravitational lensing studies are expected to play a crucial role in astrophysical research, from tight constraints on neutrino mass hierarchies to probing the physics of dark energy and testing the predictions of General Relativity on cosmological scales.

Further interdisciplinary work, coupled with advancements in computational cosmology and simulations, is expected to complement these observations, refining our understanding of the largest structures in the Universe and their evolution across cosmic time.